The horse can cover one area of 3/4 of a circle with a radius of 15m = (3/4)pi (15)^2
Additionally the horse can cover the area of two quarter-circles with a radius of 3m = pi (3)^2 / 2
So....the horse can graze in an area of pi [ 15^2 * 3/4 + 3^2 / 2 ] ≈ 544.28 m^2